Aerosol detectors are commonly used to detect the presence of aerosol particles, such as smoke, fog and/or dust particles, in the air. The most commonly used type of aerosol detector measures light that is scattered from a light beam onto a photo sensor by aerosol particles that come into contact with the light beam. This type of aerosol detector typically includes a housing that defines a chamber that allows smoke or other aerosols to enter without allowing light to enter from the outside. A light source, such as a light emitting diode (LED), is disposed within the chamber for emitting light. A detector or photo sensor, such as a photoelectric eye or photodiode, is also disposed within the chamber. In the absence of aerosol, most of the light emitted by the light source is typically absorbed by the chamber walls or some other light trap prior to reaching the detector. In this regard, the walls of the chamber are typically constructed of or treated with a non-reflecting material to minimize the light incident upon the photo sensor due to reasons other than the scattering from the light beam by aerosol particles that come into contact with the light beam. For example, the walls of the chamber may be painted a dark color, such as flat black, in order to absorb most of the light incident thereupon. If aerosol is present within the chamber, however, the light is scattered by the aerosol particles, and a portion of the scattered light is received by the detector, which can cause an alarm if the incident light exceeds a predetermined limit that is indicative of an undesirable concentration of aerosol.
Unfortunately, the conventional aerosol detector design permits a significant number of false alarms. Because conventional aerosol detectors measure the light scattered within a detection chamber, these aerosol detectors will issue an alarm anytime the photo sensor measures scattered light regardless of whether the scattered light is due to the presence of aerosol or some other contaminant. As a result, the presence of condensation, dust, fibers and insects reflects and/or scatters light, which frequently causes false alarms. For example, in certain environments, such as the cargo areas of aircraft, in which aerosol detectors are used to sense smoke, a significant amount of dirt, dust, fibers and other small particles are in the air. It is estimated that the ratio of false smoke alarms in some commercial aircraft cargo compartments to real smoke alarms is as high as 200:1. The random particles may enter the detection chamber and while the amount of light scattered by the particles may not be enough to cause an alarm initially, the particles may land on the walls of the chamber and, over time, cause enough reflection off of the chamber walls to activate a false alarm. In addition, contaminants passing through the light beam, such as insects also may cause enough reflection and/or scattering to activate a false alarm.
False alarms may be very costly. For example, a false smoke alarm in an operating aircraft may cause diversion of the aircraft, which inconveniences passengers, disrupts one or more aircraft schedules, and necessitates unscheduled maintenance on the aircraft, all of which impose significant costs on the operator of the aircraft. In other applications, such as in public buildings and homes, false alarms lead to the evacuation of the structure, automatic response by emergency personnel and damage caused by unnecessary release of automatic fire extinguishing systems, which are costly disruptions.
Thus, there is a need for an aerosol detector that is capable of accurately identifying the presence of aerosol within the detector and also capable of greatly reducing the likelihood of false alarms due to stray contaminants that may enter the detector.